Optical echoes of light near a black hole

TL;DR

This paper investigates the optical echoes produced by pulsed light sources near black holes using analogue gravity models, revealing resonance effects related to the photon sphere and offering insights into strong gravitational lensing through wave optics.

Contribution

It introduces a wave-optics analysis of optical echoes near black holes using analogue models, highlighting the role of the photon sphere and pulse duration in echo formation.

Findings

Echo tails appear when pulse duration is comparable to the photon sphere.

Resonance between incoming pulse and photon sphere causes observable signatures.

Wave-optics perspective enhances understanding of gravitational lensing phenomena.

Abstract

The light deflection under a strong gravitational field, referred to as strong gravitational lensing, provides a powerful probe of spacetime geometry. Besides, laboratory analogue models are employed to study the effects of curved spacetime and explore the design of optical devices. Here, applying the framework of analogue gravity, we reveal the behavior of the optical echo from a pulsed point-like source near a black hole, which is strongly dependent on the interplay of the black hole's photon sphere and the source's duration. We model the Schwarzschild spacetime as a Flamm paraboloid and calculate the echo response, using analytical geodesic solutions and the Huygens-Fresnel principle. Particularly, when the spatial scale of pulse duration is comparable to the photon sphere, continuous ``echo tails" appear along bright interference fringes in temporal response. Analysis in both the temporal and frequency domains reveals that these echo tails are a signature of resonance between the incoming pulse and the photon sphere. This work provides a wave-optics perspective on the interaction between dynamic sources and black holes, offering a table top window on strong gravitational lensing.